1. Field of the Invention
The present invention relates to an image decoder, an image decoding method and a program for dynamically controlling a playing quality in accordance with the frequency of a Central Processing Unit (CPU) or the remaining battery time when motion picture data is decoded and played.
2. Description of the Related Art
In recent years, when image contents are played in a portable terminal run by a battery such as a note book type personal computer or a PDA (Personal Digital Assistant), the CPU operates under a fixed frequency irrespective of the contents of the motion picture data. Accordingly, when the CPU frequency is low, the decoding operation of a part of frames is not completed within a predetermine time. Thus, a frame is undesirably dropped so that the motion picture data cannot be sometimes played with a sufficient quality. On the other hand, when the CPU frequency is high, the CPU frequency is higher than necessary. Accordingly, since the electric power of the battery is undesirably consumed more than necessary, the image contents may not be possibly played to the last due to the insufficiency of the remaining battery time.
In recent years, as CPUs mounted on the portable terminal, there are not a few CPUs that can dynamically switch the CPU frequency. For example, a CPU featuring Enhanced SpeedStep (a trademark) of Intel Corporation has two-stage modes including a maximum performance mode and a battery optimized mode that can be automatically switched depending on a computational load. However, when the CPU frequency is adjusted depending on the computational load as described above, a computational load monitor mechanism for monitoring the computational load of the CPU is required. Therefore, the computational load monitor mechanism consumes an electric power to insufficiently save the electric power.
Thus, in Japanese Patent Application Laid Open No. 2003-280760, a technique is proposed that a computational load monitor mechanism for monitoring the computational load of a CPU is not provided and the CPU frequency is adjusted on the basis of the length of each frame data of motion picture data encoded with an MPEG (Moving Picture Experts Group) standard. According to the technique disclosed in this Patent Document, when the length of the frame data is short, the CPU frequency is adjusted to be low so that a consumed power can be suppressed. When the length of the frame data is long, the CPU frequency is adjusted to be high so that the data can be completely decoded within a predetermined time.
In the above-described technique, the CPU frequency can be dynamically controlled depending on the contents of the motion picture data. However, the playing quality of the motion picture data cannot be dynamically controlled in accordance with the CPU frequency or the remaining battery time. Accordingly, the technique disclosed in the Patent Document cannot supports a request of a user who wants to enjoy image contents with a high quality, for instance, when there is enough power for processing in the CPU.